1. Field of the Invention
Aspects of the present invention relate to an apparatus and method of estimating the quality of an input signal, and an optical disc driver including the apparatus for estimating the quality of an input signal.
2. Description of the Related Art
An input signal is an analog signal, such as a radio frequency (RF) signal, that is reproduced from a storage medium. For example, a disc is a storage medium that stores a binary signal, though an RF signal read from the disc has the properties of an analog signal due to the characteristics of the disc and the optical characteristics of an optical disc driver driving the disc. Hence, the optical disc driver may perform a binarization process to change the RF signal to a binary signal. A binarization process may be performed using a comparator 100 as illustrated in FIG. 1.
FIG. 1 is a functional block diagram illustrating a general binarization process. Referring to FIG. 1, the general binarization process is performed using the comparator 100 and a low-pass filter 110. The comparator 100 compares an input RF signal with a slicing level and outputs the result of the comparison. The input RF signal is read from a disc. The output of the comparator 100 is simultaneously transmitted to the low-pass filter 110 and another processing unit (not shown). The low-pass filter 110 low-pass filters the output of the comparator 100. An output of the low-pass filter 110 is transmitted as the slicing level to the comparator 100.
An existing optical disc driver converts the RF signal read from the disc into a binary signal using the binarization process illustrated in FIG. 1, makes a system clock by applying the binary signal to a phase locked loop, and plays back data read from the disc using the binary signal and the system clock. There is a slight difference, or a jitter, between the phases of the RF signal and the system clock.
FIGS. 2A through 2C illustrate a jitter generated between an offset-removed RF signal and a system clock based on a falling edge of the system clock. In FIGS. 2A through 2C, in an ideal case, a falling edge of the system clock precisely meets a zero crossover point of the RF signal. However, in actuality, the falling edge of the system clock does not precisely meet the zero crossover point of the RF signal, and there is a slight temporal difference between them. This difference is referred to as a jitter.
In an existing technique, a jitter corresponding to the difference between the RF signal and the system clock is used to estimate the quality of the RF signal. In other words, in an ideal case, a jitter is hardly measured because an edge of the system clock precisely meets a zero crossover point of the RF signal. However, when the RF signal is affected by noise or abnormal circumstances, the edge of the system clock is not precisely overlapped by the zero crossing point of the RF signal, and the jitter is therefore measured. Thus, the quality of the RF signal can be estimated based on the measured jitter value.
However, as the recording density of a disc increases, the magnitude of an RF signal corresponding to a binary signal with a short T (where T denotes one pit interval) decreases. Accordingly, even when a small amount of noise is added to the RF signal corresponding to a binary signal with a short period, the RF signal is relatively greatly distorted or the RF signal is near the zero crossing point. Consequently, a wrong jitter value may be measured. Therefore, it the qualities of RF signals read out from high-density discs cannot be estimated by using jitter values measured based on the differences between the RF signals and system clocks.